I got Harvest and the life eater node, 20 intel, an 200% healing effectiveness.
A Harvest hit give me 20 health back (tested on dummy) so healing effectiveness doesn’t apply.
Reaper form + Reap gives me 45 without Harbringer of blood (15 base healt +200%) : healing effectiveness works.
It seems Healing Effectiveness doesn’t affect the Life Eater while it seems to affect health gaind on hit with reap.
Tooltip on Healing effectiveness says it doesn’t affect heal on hit so something may be wrong, either the descriptions of some nodes, Reap or Life Eater.